Round milling inserts for milling purposes have the advantage in comparison with polygonal milling inserts having straight cutting edges in that they lack fragile corners. For this reason, the same are advantageously used in applications where the milling cutter should be able to mill off or remove large amounts of material from the work piece without considerable requirements of surface smoothness of the generated surface. The fact that the round milling inserts as a consequence of their arched cutting edges giving rise to wave formations in the generated surface is accordingly incidental in comparison with the capacity of the milling inserts to remove deep material layers from the work piece. Round milling inserts are found in single sided as well as double-sided embodiments, the first-mentioned ones of which are formed with cutting edges only along the upper sides and therefore can be given a positive insert macro geometry, while the insert macro geometry of the last-mentioned ones is negative as a consequence of cutting edges being formed along the upper side as well as the underside.
The invention only concerns itself with the first-mentioned type of milling inserts, i.e., single sided, indexable milling inserts having a round basic shape and positive insert macro geometry.
Generally, milling inserts having a positive insert macro geometry are more easy-cutting, concerning generated cutting force, than double-sided, negative milling inserts because the cutting edge is introduced under the chip so as to, like a knife or wedge, lift out and cut the chip rather than pushing the same in front of itself during shearing. Nevertheless, also single sided, round milling inserts are subjected to considerable cutting forces, among other things as a consequence of the chip obtaining an increasing thickness with increasing cutting depths. From a narrow end, the thickness of the chip increases to a maximum value, the absolute value of which depends on several factors, but above all the feed rate of the milling cutter per milling insert in the milling cutter body.
In order to strengthen the cutting edge of milling inserts in general, the same is usually formed with a chamfer surface in the transition between the clearance surface and the chip surface. In previously known inserts having a round basic shape, said reinforcing chamfer surface is of a uniform width along the entire periphery of the upper side (i.e., 360°). See, for instance, US 2009/0290946 A1. This means that the cutting edge along its entire operative arc length has one and the same geometry and strength irrespective of cutting depth and thereby irrespective of the shape of the generated chips. The cutting edge is as obtuse in the area where the chip is thin, as in the area where the same is thick. For this reason, the cutting forces will be unnecessary large at the same time as the wear-out of the cutting edge becomes uneven. To this, also the fact contributes that the known, round milling inserts have one and the same nominal clearance angle along their entire circumferential, conical clearance surface, which means that the functional clearance angles, when the milling insert is mounted in the basic body, will vary and give rise to local temperature rises in the segments where the real clearance from the red-hot material of the work piece becomes too small. Thus, the consequence of the same nominal clearance angle becomes an impaired service life of the milling insert.